System and/or Method for Determining Sufficiency of Pseudorange Measurements

ABSTRACT

The subject matter disclosed herein relates to a system and method for determining a sufficiency of measurements for locating positions. In one example, although claimed subject matter is not so limited, a process to improve accuracy of pseudorange measurements may be terminated in response to a weighting of quantitative assessments of at least some of such pseudorange measurements.

This application claims the benefit of and is a non-provisional ofco-pending U.S. Provisional Application Ser. No. 60/802,020 filed on May19, 2006, entitled “An Improved Measurement Sufficiency Test for GPSSearches Using a Plurality of Search Modes” which is assigned to theassigner hereof and which is hereby expressly incorporated by referencein its entirety for all purposes.

BACKGROUND

1. Field

The subject matter disclosed herein relates to determining a location ofa position based upon signals received from geo-location satellites.

2. Information

A satellite positioning system (SPS) typically comprises a system ofearth orbiting satellites enabling entities to determine their locationon the earth based, at least in part, on signals received from thesatellites. Each such SPS satellite typically transmits a signal markedwith a repeating pseudo-random noise (PN) code of 1,023 chipsdistinguishing the satellite from other SPS satellites where the 1,023chips repeat every millisecond. The signal is also typically modulatedwith data bits, where each data bit has a 20 ms duration in themodulated signal.

FIG. 1 illustrates a typical application of a geo-location system,whereby a subscriber station 100 in a wireless communications systemreceives transmissions from satellites 102 a, 102 b, 102 c, 102 d in theline of sight to subscriber station 100, and derives time measurementsfrom four or more of the transmissions. Subscriber station 100 providesthe measurements to position determination entity (PDE) 104, whichdetermines the position of the station from the measurements.Alternatively, the subscriber station 100 may determine its own positionfrom this information.

Subscriber station 100 may search for a transmission from a particularsatellite by correlating the PN code for the satellite with a receivedsignal. The received signal typically comprises a composite oftransmissions from one or more satellites within a line of sight to areceiver at station 100 in the presence of noise. A correlation may beperformed over a range of code phase hypotheses known as the code phasesearch window W_(CP), and over a range of Doppler frequency hypothesesknown as the Doppler search window W_(DOPP). Such code phase hypothesesare typically represented as a range of PN code shifts while suchDoppler frequency hypotheses are typically represented as Dopplerfrequency bins.

A correlation is typically performed over an integration time “I” whichmay be expressed as the product of N_(c) and M, where N_(c), is thecoherent integration time, and M is number of coherent integrationswhich are non-coherently combined. For a particular PN code, correlationvalues are typically associated with corresponding PN code shifts andDoppler bins to define a two-dimensional correlation function. Peaks ofthe correlation function are located and compared to a predeterminednoise threshold. The threshold is typically selected so that the falsealarm probability, the probability of falsely detecting a satellitetransmission, is at or below a predetermined value. A time measurementfor the satellite is typically derived from a location of an earliestnon-side lobe peak along the code phase dimension which equals orexceeds the threshold. A Doppler measurement for the subscriber stationmay be derived from the location of the earliest non-side lobe peakalong the Doppler frequency dimension which equals or exceeds thethreshold.

Current subscriber station architectures place significant constraintson the process of searching for location determination signals. In ashared RF architecture, for example, core RF circuitry in the subscriberstation is typically shared between a location determination receivepath, and voice/data communication transmit and receive paths.Accordingly, employing such a shared RF architecture in an SPS functionmay diminish an ability of such a shared architecture to perform avoice/data communication function or other function sharing commonresources. Accordingly, there is a desire to reduce use of such commonresources for determining locations of position.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive embodiments will be described withreference to the following figures, wherein like reference numeralsrefer to like parts throughout the various figures unless otherwisespecified.

FIG. 1 is a schematic diagram of a geo-location system according to anembodiment.

FIG. 2 is a flow diagram illustrating a process for determining aposition of a receiver from a geo-location system according to anembodiment.

FIG. 3 is a schematic diagram of a two-dimensional domain to be searchedfor detection of a signal transmitted from a space vehicle according toan embodiment.

FIG. 4 illustrates an overlap by a prescribed number of chips in asearch window to avoid missing peaks that appear at segment boundariesaccording to an embodiment.

FIG. 5 is a schematic diagram of a system for processing signals todetermine a position location according to an embodiment.

FIG. 6 is a schematic diagram of a subscriber station according to anembodiment.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of claimed subject matter. Thus, theappearances of the phrase “in one embodiment” or “an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in one or moreembodiments.

The methodologies described herein may be implemented by various meansdepending upon applications according to particular embodiments. Forexample, such methodologies may be implemented in hardware, firmware,software, and/or combinations thereof. In a hardware implementation, forexample, a processing unit may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,electronic devices, other devices units designed to perform thefunctions described herein, or a combination thereof.

“Instructions” as referred to herein relate to expressions whichrepresent one or more logical operations. For example, instructions maybe “machine-readable” by being interpretable by a machine for executingone or more operations on one or more data objects. However, this ismerely an example of instructions and claimed subject matter is notlimited in this respect. In another example, instructions as referred toherein may relate to encoded commands which are executable by aprocessing circuit having a command set which includes the encodedcommands. Such an instruction may be encoded in the form of a machinelanguage understood by the processing circuit. Again, these are merelyexamples of an instruction and claimed subject matter is not limited inthis respect.

“Storage medium” as referred to herein relates to media capable ofmaintaining expressions which are perceivable by one or more machines.For example, a storage medium may comprise one or more storage devicesfor storing machine-readable instructions and/or information. Suchstorage devices may comprise any one of several media types including,for example, magnetic, optical or semiconductor storage media. Suchstorage devices may also comprise any type of long term, short term,volatile or non-volatile devices memory devices. However, these aremerely examples of a storage medium and claimed subject matter is notlimited in these respects.

Unless specifically stated otherwise, as apparent from the followingdiscussion, it is appreciated that throughout this specificationdiscussions utilizing terms such as “processing,” “computing,”“calculating,” “selecting,” “forming,” “enabling,” “inhibiting,”“locating,” “terminating,” “identifying,” “initiating,” “weighting,”“obtaining,” “hosting,” “maintaining,” “representing,” “estimating,”“receiving,” “transmitting,” “determining” and/or the like refer to theactions and/or processes that may be performed by a computing platform,such as a computer or a similar electronic computing device, thatmanipulates and/or transforms data represented as physical electronicand/or magnetic quantities and/or other physical quantities within thecomputing platform's processors, memories, registers, and/or otherinformation storage, transmission, reception and/or display devices.Such actions and/or processes may be executed by a computing platformunder the control of machine-readable instructions stored in a storagemedium, for example. Such machine-readable instructions may comprise,for example, software or firmware stored in a storage medium included aspart of a computing platform (e.g., included as part of a processingcircuit or external to such a processing circuit). Further, unlessspecifically stated otherwise, process described herein, with referenceto flow diagrams or otherwise, may also be executed and/or controlled,in whole or in part, by such a computing platform.

A “space vehicle” (SV) as referred to herein relate to an object that iscapable of transmitting signals to receivers on the earth's surface. Inone particular embodiment, such an SV may comprise a geostationarysatellite. Alternatively, an SV may comprise a satellite traveling in anorbit and moving relative to a stationary position on the earth.However, these are merely examples of SVs and claimed subject matter isnot limited in these respects.

A “location” as referred to herein relates to information associatedwith a whereabouts of an object or thing according to a point ofreference. Here, for example, such a location may be represented asgeographic coordinates such as latitude and longitude. Alternatively,such a location may be represented as a street address, municipality orother governmental jurisdiction, postal zip code and/or the like.However, these are merely examples of how a location may be representedaccording to particular embodiments and claimed subject matter is notlimited in these respects.

Location determination techniques described herein may be used forvarious wireless communication networks such as a wireless wide areanetwork (WWAN), a wireless local area network (WLAN), a wirelesspersonal area network (WPAN), and so on. The term “network” and “system”may be used interchangeably herein. A WWAN may be a Code DivisionMultiple Access (CDMA) network, a Time Division Multiple Access (TDMA)network, a Frequency Division Multiple Access (FDMA) network, anOrthogonal Frequency Division Multiple Access (OFDMA) network, aSingle-Carrier Frequency Division Multiple Access (SC-FDMA) network, andso on. A CDMA network may implement one or more radio accesstechnologies (RATs) such as cdma2000, Wideband-CDMA (W-CDMA), to namejust a few radio technologies. Here, cdma2000 may include technologiesimplemented according to IS-95, IS-2000, and IS-856 standards. A TDMAnetwork may implement Global System for Mobile Communications (GSM),Digital Advanced Mobile Phone System (D-AMPS), or some other RAT. GSMand W-CDMA are described in documents from a consortium named “3rdGeneration Partnership Project” (3GPP). Cdma2000 is described indocuments from a consortium named “3rd Generation Partnership Project 2”(3GPP2). 3GPP and 3GPP2 documents are publicly available. A WLAN maycomprise an IEEE 802.11x network, and a WPAN may comprise a Bluetoothnetwork, an IEEE 802.15x, for example. Such location determinationtechniques described herein may also be used for any combination ofWWAN, WLAN and/or WPAN.

According to an embodiment, a device and/or system may estimate itslocation based, at least in part, on signals received from SVs. Inparticular, such a device and/or system may obtain “pseudorange”measurements comprising approximations of distances between associatedSVs and a navigation satellite receiver. In a particular embodiment,such a pseudorange may be determined at a receiver that is capable ofprocessing signals from one or more SVs as part of a SatellitePositioning System (SPS). Such an SPS may comprise, for example, aGlobal Positioning System (GPS), Galileo, Glonass, a system that usessatellites from a combination of these systems, or any SPS developed inthe future. As used herein, an SPS will also be understood whereappropriate to include pseudolite systems—ground-based transmitters thatbroadcast a PN code or other ranging code (similar to a GPS or CDMAcellular signal), or systems that use a combination of satellites andpseudolites. To determine its position, a satellite navigation receivermay obtain pseudorange measurements to three or more satellites as wellas their positions at time of transmitting. Knowing the SVs' orbitalparameters, these positions can be calculated for any point in time. Apseudorange measurement may then be determined based, at least in part,on the time a signal travels from an SV to the receiver, multiplied bythe speed of light. While techniques described herein may be provided asimplementations of location determination in a GPS type of SPS asspecific illustrations according to particular embodiments, it should beunderstood that these techniques may also apply to other types of SPS',and that claimed subject matter is not limited in this respect.

According to an embodiment, a pseudorange measurement may comprise asingle measurement of a distance between a navigation receiver and anSV. Here, accordingly, accuracy of an initial pseudorange measurement toa particular SV may be improved by obtaining one or more subsequentpseudorange measurements to replace and/or statistically combine withinitial pseudorange measurement. Therefore, in this context, such apseudorange measurement may also comprise a distance between anavigation receiver and an SV estimated based, at least in part, uponmultiple individual pseudorange measurements taken.

The accuracy of an estimate of a location based on pseudorangemeasurements may be determined, at least in part, on accuracy of thepseudorange measurements. According to an embodiment, although claimedsubject matter is not limited in this respect, a satellite receiver maycontinue attempting to improve the accuracy associated with pseudorangemeasurements until such a location may be estimated with sufficientaccuracy. After a satellite receiver has estimated a location withsufficient accuracy, it may cease attempting to improve accuracy of thepseudorange measurements and employ resources used for obtainingpseudorange measurements to another function.

A “quantitative assessment of accuracy” as referred to herein relates toa quantitative metric associated with accuracy of an estimate of avalue. For example, such a quantitative assessment of accuracy maycomprise a variance associated with an estimate of a value based uponone or more measurements. In another embodiment, such a quantitativeassessment of accuracy may comprise a root mean square error associatedwith an estimate of a value based upon one or more measurements. In aparticular embodiment, although claimed subject matter is not limited inthis respect, such a quantitative assessment of accuracy may relate to aquantitative metric associated with accuracy of a pseudorangemeasurement. However, these are merely examples of a quantitativeassessment of accuracy and claimed subject matter is not limited inthese respects.

While accuracy of pseudorange measurements may affect the accuracy of anestimated location derived from such pseudorange measurements, locationsof SVs used for generating such pseudorange measurements may also affectthe accuracy of such a location estimate. In a particular embodiment,locations of SVs in orbit with respect to one another and/or withrespect to a receiver may provide “geometries” affecting accuracy of anestimated location of the receiver based, at least in part, onpseudorange measurements to said SVs. In one embodiment, such geometriesmay be expressed, at least in part, by approximated azimuth andelevation angles of locations of SVs with respect to a location of areceiver. However, this is merely an example of how such geometries maybe expressed according to a particular embodiment and claimed subjectmatter is not limited in this respect.

Briefly, one embodiment relates to a process of attempting to improveaccuracy of pseudorange measurements for estimating a location of aposition. Quantitative assessments of accuracy associated with aplurality of pseudorange measurements to SVs used for estimating such aposition of a location may be weighted according to geometriesassociated with the SVs. The process of attempting to improve theaccuracy of the pseudorange measurements may then be terminated orexited in response to the weighted quantitative assessments. However,this is merely sample embodiment an claimed subject matter is notlimited in this respect.

In an alternative embodiment, pseudorange measurements to space vehiclesSVs at a location may be determined using a first search dwell. A secondsearch dwell may be selectively employed to increase the number ofpseudorange measurements and/or increase the accuracy of the pseudorangemeasurements in response to at least some quantitative assessmentsassociated with the pseudorange measurements which are weightedaccording to geometries associated with the SVs. Again, this is merelyan example embodiment and claimed subject matter is not limited in thisrespect.

FIG. 2 is a flow diagram illustrating a process 200 for estimating alocation of a position from a geo-location system according to anembodiment. In one embodiment, process 200 may be performed by asubscriber device comprising a receiver to receive signals from SVs.Such a subscriber device may be in communication with a PDE overterrestrial wireless communication link, for example. Such a subscriberdevice may employ common processing resources for determining a locationof its position based, at least in part, on signals received from SVsand for performing tasks and/or functions such as, for example, voiceand/or data communications over wireless communication links. Such adevice may employ at least some of its processing resources during“dwells” to estimate its location from signals received from SVs.Processing during such dwells may provide pseudorange measurements toSVs for use in estimating location using techniques known to those ofordinary skill in the art. In a particular embodiment, although claimedsubject matter is not limited in this respect, if a device estimates itslocation with sufficient accuracy following performing a first dwell atblock 204, the device may not perform a second dwell at block 220 toestimate its position with greater accuracy. By not employing the seconddwell, such a subscriber device may be able to deploy common processingresources to perform other platform functions instead and shorten thetime required to obtain a location fix.

In an alternative embodiment, portions of process 200 may be performedby a device in communication with a subscriber station such as, forexample, a PDE. For example, a subscriber station may providepseudorange measurements to a PDE (e.g., over a terrestrial wirelesscommunication link) while the PDE estimates the location of thesubscriber station based, at least in part, on the received pseudorangemeasurements. Also, such a PDE may employ portions of process 200 todetermine whether the subscriber station is to perform a second dwell atblock 220 as discussed below, and signal to the subscriber stationaccordingly.

Process 200 may commence at 202 in response any one of several eventssuch as, for example, acquisition of a signal from a base station at asubscriber station, an input from a user interface or initiation of anemergency “911” call, to name just a few. Block 204 may compriseperforming a first dwell to obtain pseudorange measurements based, atleast in part, upon signals received from SVs. According to a particularembodiment, and as discussed below with reference to FIGS. 3 and 4, suchpseudorange measurements may be based, at least in part, on acorrelation of a range of code phase hypotheses, providing a code phasesearch window, and a range of Doppler frequency hypotheses, providing aDoppler search window. However, this is merely an example of howpseudorange measurements may be obtained from a dwell according to aparticular embodiment and claimed subject matter is not limited in thisrespect.

According to an embodiment, a dwell may employ a particular “searchmode” for obtaining a pseudorange measurement characterized by a set ofsystem parameters. As illustrated in Table 1 below in connection with aparticular example, such a set of system parameters characterizing aparticular search mode may comprise one or more of a frequency searchband, number of frequency bins, number of code bins, coherentintegration time, number of non-coherent integration segments, totalintegration time, detection threshold (e.g., based on C/N_(o)) andsensitivity (e.g., based on C/N_(o)). However, these are merely examplesof system parameters that may characterize a search mode and claimedsubject matter is not limited in this respect. In a particularembodiment, dwells performed at blocks 204 and 220 may employ the samesearch mode, but may employ different search modes in differentembodiments depending on particular implementation preferences.

It should be understood that signals received from some SVs at a devicemay be stronger than signals received from other SVs due to, for exampledifferent transmission power levels being employed at different SVs,blockage of certain signals by physical barriers and different distancesbetween the device and different SVs. In the detection of PN codesignals from an SV, block 206 may employ signal processing to remove thepresence of cross-correlations resulting from PN code signalstransmitted by other SVs.

According to an embodiment, although claimed subject matter is notlimited in this respect, there are a finite number (“P”) of SVs that maybe within the line of sight or otherwise “visible” to a device. In aparticular embodiment, although claimed subject matter is not limited inthis respect, at block 208 a subscriber device may determine P from anacquisition assistance (AA) message received from a PDE over aterrestrial wireless communication link, for example. Such a subscriberdevice may also identify a number of SVs (“Q”) among the set of P SVsfrom which sufficiently strong measurements have been produced duringthe first dwell at block 204. In one embodiment, block 208 may determinesuch a strength of a measurement based, at least in part, on asignal-to-noise ratio resulting from a signal detected from anassociated SV as illustrated in U.S. Pat. No. 6,873,910 to Rowitch etal. titled “Procedure for Searching for Position Determination SignalsUsing a Plurality of Search Modes.” In an alternative embodiment, such astrength of a measurement may be based, at least in part, on aquantitative assessment of accuracy associated with the measurement.However, these are merely examples of how a strength of a pseudorangemeasurement may be determined according to particular embodiments andclaimed subject matter is not limited in this respect.

If the number of SVs “Q” for which a device has produced sufficientlystrong measurement equals the number of SVs visible to the device P, asdetermined at diamond 210, process 200 may terminate or exit at 224 suchthat a second dwell is not performed at block 220. Otherwise, diamond202 may initiate such a second dwell at block 220 if Q is less than aminimum number MIN_Q of desired measurements.

If Q meets or exceeds MIN_Q as determined at diamond 212, blocks 214through 218 may determine a quantitative location accuracy metric based,at least in part, on quantitative assessments of accuracy of pseudorangemeasurements obtained from at least some of the SVs visible to thedevice and geometries associated with the SVs. Such a location accuracymetric may then be compared with a threshold value THRESHOLD at diamond222 to determine whether to perform a second dwell at block 220. Asshown in the particular illustrated embodiment, process 200 mayterminate a process to improve the accuracy of pseudorange measurementsby not performing a second dwell at block 220 in response to acomparison of the location accuracy metric to THRESHOLD at diamond 222.

According to a particular embodiment, although claimed subject matter isnot limited in this respect, a location accuracy metric (e.g., to becompared with THRESHOLD at diamond 222), may be based, at least in part,on quantitative assessments of accuracy associated with pseudorangemeasurements obtained from a first dwell at block 204. In particular,such a location accuracy metric may be based, at least in part, on suchquantitative assessments which are weighted according to geometriesassociated with SVs used to determine the pseudorange measurements. In aparticular embodiment, although claimed subject matter is not limited inthis respect, block 218 may determine such a location accuracy metric asa weighted horizontal dilution of precision (WHDOP) expressed inrelations (1) and (2) as follows:

WHDOP=√{square root over (Tr(Σ_(xy)))}  (1)

Σ_(xy)=[(G ^(T) WG)⁻¹]_(2×2)  (2)

Where:

G comprises a matrix quantifying affects of satellite geometries on theaccuracy of a position location estimate; and

W comprises a matrix having elements based, at least in part, on aquantitative assessments of accuracy of pseudorange measurements usedfor determining the position location estimate.

The operator [:]_(2×2) takes the 2×2 upper left submatrix of theargument (e.g., (G^(T)WG)⁻¹ may comprise a 4×4 matrix in a particularembodiment). At block 214, the matrix G may be given by:

$G = \begin{bmatrix}{{\cos \left( \alpha_{1} \right)}{\sin \left( \beta_{1} \right)}} & {{\cos \left( \alpha_{1} \right)}{\cos \left( \beta_{1} \right)}} & {\sin \left( \alpha_{1} \right)} & 1 \\{{\cos \left( \alpha_{2} \right)}{\sin \left( \beta_{2} \right)}} & {{\cos \left( \alpha_{2} \right)}{\cos \left( \beta_{2} \right)}} & {\sin \left( \alpha_{2} \right)} & 1 \\\vdots & \vdots & \vdots & \vdots \\{{\cos \left( \alpha_{N} \right)}{\sin \left( \beta_{N} \right)}} & {{\cos \left( \alpha_{N} \right)}{\cos \left( \beta_{N} \right)}} & {\sin \left( \alpha_{N} \right)} & 1\end{bmatrix}$

where α_(i)=elevation of SV_(i), β_(i)=azimuth of SV_(i) for SV₁, SV₂, .. . SV_(N) visible to a receiver at a device. Here, according to aparticular embodiment, α_(i) and β_(i) may be received from a PDE orother device in an AA message over a terrestrial wireless communicationlink. Alternatively, a device may independently determine such anglesfrom signals from main lobe detections at a device antenna using phasedarray signal processing, for example. However, these are merely examplesof how a device may obtain information descriptive of geometriesassociated with SVs and claimed subject matter is not limited in theserespects.

According to a particular embodiment, a quantitative assessment ofaccuracy associated with a pseudorange measurement may comprise avariance associated with such a pseudorange measurement. Here, block 214may determine matrix W as follows:

$W = \begin{bmatrix}{1/\sigma_{1}^{2}} & 0 & \ldots & 0 \\0 & {1/\sigma_{2}^{2}} & 0 & \vdots \\\vdots & 0 & \ddots & 0 \\0 & \ldots & 0 & {1/\sigma_{N}^{2}}\end{bmatrix}$

where σ² ₁, σ² ₂, . . . σ² _(N),=variances of pseudorange measurementsassociated with SV₁, SV₂, . . . SV_(N) visible to a receiver at adevice.

In an alternative embodiment, a quantitative assessment of accuracyassociated with a pseudorange measurement may comprise a root meansquare error (RMSE) associated with the pseudorange measurement. In thisalternative embodiment, block 214 may determine matrix W as follows:

$W = \begin{bmatrix}{1/{RMSE}_{1}^{2}} & 0 & \ldots & 0 \\0 & {1/{RMSE}_{2}^{2}} & 0 & \vdots \\\vdots & 0 & \ddots & 0 \\0 & \ldots & 0 & {1/{RMSE}_{N}^{2}}\end{bmatrix}$

where RMSE₁, RMSE₂, . . . RMSE_(N) comprise root mean square errorsassociated with pseudorange measurements associated with SV₁, SV₂, . . .SV_(N) visible to a receiver at a device.

Here, block 214 may access a pre-computed value for RMSE_(i) from alook-up table in memory indexed by an elevation angle of SV_(i) (e.g.,α_(i)) and an assessment of the strength of the signal such as C/N_(o),where C represents a strength of PN code signals received from SV_(i)and N_(o) represents a noise power at a receiver. However, this ismerely an example of how a root mean square error may be obtained fordetermining WHDOP according to a particular embodiment and claimedsubject matter is not limited in this respect.

It should be understood that depending on values of particular elementsin matrices G and W, the expression (G^(T)WG)⁻¹ may provide anundetermined result if the expression G^(T)WG does not provide aninvertible matrix. Accordingly, if block 216 fails to compute(G^(T)WG)⁻¹ (e.g., because the expression G^(T)WG is not invertible),process 200 may initiate performing a second dwell at block 220.

It should be observed that WHDOP, in the particular embodiment accordingto relation (1), sets forth a measure of horizontal error distributionand has units in linear length such as meters. Accordingly, a value forTHRESHOLD to be compared with WHDOP at diamond 222 may be similarlyexpressed in units of linear length. In a particular embodiment whereerrors in pseudorange measurements are Gaussian distributed, horizontalposition components may comprise Gaussian random variables. However, anoverall horizontal error (e.g., (East_error²+North_error²)^(1/2)) mayhave a chi-squared distribution with two degrees of freedom wherecontours of equal probability are ellipsoidal. Nevertheless, a circularerror probable model may approximate such a distribution as follows:

CEP₅₀≈0.75·WHDOP

CEP₉₅≈2.00·WHDOP

where CEP defines a radius of a circle capturing X % of an errordistribution when centered at the correct location. In a particularembodiment, a performance requirement may specify a particular radius(e.g., in linear length units) for a particular CEP_(X). Accordingly, avalue for THRESHOLD to be compared with WHDOP at diamond 222 may bedetermined for a desired radius about center and Z % center errorprobable according to empirical relationships between CEP and WHDOP.

According to an embodiment, a particular value for THRESHOLD may beselected based, at least in part, on a trade-off between accuracy of alocation estimate and time to fix (TTF) such an estimate. Depending onseveral factors such as the strength of signals received from SVs andnoise, reducing a tolerance for accuracy in a location estimate mayincrease a required TTF the location estimate. Likewise, increasing atolerance for accuracy in a location estimate may reduce a TTF thelocation estimate. In particular embodiments, although claimed subjectmatter is not limited in this respect, a value for THRESHOLD may beselected based, at least in part, on experimental measurementsevaluating accuracy of location estimates versus TTF under a variety ofconditions.

In one embodiment, a value of THRESHOLD may be selected to optimizerelationships among probabilistic variables such as, for example, aprobability of not terminating early although pseudorange measurementaccuracy is sufficient without a second dwell at block 220, aprobability of terminating early and forgoing a second dwell at block220 even though pseudorange measurement accuracy is not sufficientwithout the second dwell at block 220, and a probability of correctlydeciding to either perform the second dwell at block 220 or forgoing thesecond dwell if not needed. However, these are merely examples ofprobabilistic variables that may be optimized in determining a value ofTHRESHOLD for a particular embodiment and claimed subject matter is notlimited in this respect.

As pointed out above, although claimed subject matter is not limited inthis respect, dwells performed at blocks 204 and 220 may process signalsreceived at a receiver according to a two-dimensional domain asillustrated in FIG. 3. As illustrated in the aforementioned U.S. Pat.No. 6,873,910, such a two-dimensional domain or “window” may be searchedfor detection of a signal transmitted from an SV to determine apseudorange measurement to the SV.

According to an embodiment, an SV which is likely to be visible at areceiver (e.g., as indicated in an AA message) may be associated with aparticular set of search window parameters defining a two-dimensionaldomain of code phase and Doppler frequency hypotheses to be searched forthe SV. In one implementation, illustrated in FIG. 3, search windowparameters for an SV comprise a code phase search window size,WIN_SIZE_(CP), a code phase window center, WIN_CENT_(CP), a Dopplersearch window size, WIN_SIZE_(DOPP), and a Doppler window center,WIN_CENT_(DOPP). In the case where the entity whose position is soughtto be determined is a subscriber station in an IS-801 compliant wirelesscommunication system, these parameters may be indicated by an AA messageprovided to the subscriber station by a PDE.

The two-dimensional search space for an SV illustrated in FIG. 3 shows acode phase axis is a horizontal axis, and a Doppler frequency axis as avertical axis, but this assignment is arbitrary and could be reversed.The center of the code phase search window is referred to asWIN_CENT_(CP), and the size of the code phase search window is referredto as WIN_SIZE_(CP). The center of the Doppler frequency search windowis referred to as WIN_CENT_(DOPP), and the size of the Doppler frequencysearch window is referred to as WIN_SIZE_(DOPP).

As pointed out above dwells at blocks 204 and 220 may be formedaccording to any one of several “search modes” setting forth systemparameters tailored to particular performance desired and/or particularprocessing resources available for a search dwell. Depending on aparticular search mode, and as illustrated in FIG. 3, a search space maybe partitioned into a plurality of segments 1202 a, 1202 b, 1202 c, eachof which is characterized by a range of Doppler frequencies and a rangeof code phases. In one example, illustrated in Table 1 below, a range offrequencies associated with a segment is +/−250 Hz for search modes 0,1, and 2, and is +/−62.5 Hz for search mode 3, and the range of codephases associated with a segment is thirty-two chips. In this particularexample, a range of frequencies characterizing a segment is divided upinto twenty bins, and the range of code phases characterizing a segmentis divided into sixty-four bins.

TABLE 1 Freq. Coh. Total Det. Search # # Integ. Non- Integ. Thresh.Sensitivity Band Freq. Code Time Coh. Time C/N_(o) C/N_(o) Mode (Hz)Bins Bins (ms) Integ. (ms) (dB-Hz) (dB-Hz) 0 +/−250 20 64 20 1 20 29.831.0 1 +/−250 20 64 20 4 80 25.0 26.4 2 +/−250 20 64 20 44 880 18.1 19.24 +/−62.5 20 64 80 22 1760 14.0 15.45

According to an embodiment, a range of code phases characterizing asegment may be equal to the capacity of a channel of a correlator tosearch of the segment through a single channel pass. In one particularexample where the channel capacity is thirty-two chips, for example, arange of code phases characterizing a segment may be likewise thirty-twochips, but it should be appreciated that other examples are possible.

Segments may be caused to overlap by a prescribed number of chips toavoid missing peaks that appear at segment boundaries as illustrated inFIG. 4. Here, a tail end of segment 1202 a overlaps the front end ofsegment 1202 b by Δ chips, and the tail end of segment 1202 b likewiseoverlaps the front end of segment 1202 c by Δ chips. Because of theoverhead due to this overlap, an effective range of code phasesrepresented by a segment may be less than the channel capacity. In thecase where the overlap is four chips, for example, an effective range ofcode phrases represented by a segment may be twenty-eight chips.

A system for searching for position determination signals within aprescribed time period is illustrated in FIG. 5 according to aparticular embodiment. However, this is merely an example of a systemthat is capable of searching for position determination signalsaccording to a particular embodiment and other systems may be usedwithout deviating from claimed subject matter. As illustrated in FIG. 6according to a particular embodiment, such a system may comprise acomputing platform including a processor 1302, memory 1304, andcorrelator 1306. Correlator 1306 may be adapted to produce correlationfunctions from signals provided by a receiver (not shown) to beprocessed by processor 1302, either directly or through memory 1304.Correlator 1306 may be implemented in hardware, software, or acombination of hardware and software. However, these are merely examplesof how a correlator may be implemented according to particularembodiments and claimed subject matter is not limited in these respects.

According to an embodiment, memory 1304 may store machine-readableinstructions which are accessible and executable by processor 1302 toprovide at least a portion of a computing platform. Here, processor 1302in combination with such machine-readable instructions may be adapted toperform all or portions of process 200 illustrated above with referenceto FIG. 2. In a particular embodiment, although claimed subject matteris not limited in these respects, processor 1302 may direct correlator1306 to search for position determination signals as illustrated aboveand derive measurements from correlation functions generated bycorrelator 1306.

Returning to FIG. 5, radio transceiver 1406 may be adapted to modulatean RF carrier signal with baseband information, such as voice or data,onto an RF carrier, and demodulate a modulated RF carrier to obtain suchbaseband information. An antenna 1410 may be adapted to transmit amodulated RF carrier over a wireless communications link and receive amodulated RF carrier over a wireless communications link.

Baseband processor 1408 may be adapted to provide baseband informationfrom CPU 1402 to transceiver 1406 for transmission over a wirelesscommunications link. Here, CPU 1402 may obtain such baseband informationfrom an input device within user interface 1416. Baseband processor 1408may also be adapted to provide baseband information from transceiver1406 to CPU 1402 for transmission through an output device within userinterface 1416.

User interface 1416 may comprise a plurality of devices for inputting oroutputting user information such as voice or data. Such devices mayinclude, for example, a keyboard, a display screen, a microphone, and aspeaker.

GPS receiver 1412 may be adapted to receive and demodulate GPS satellitetransmissions, and provide the demodulated information to correlator1418.

Correlator 1418 may be adapted to derive GPS correlation functions fromthe information provided to it by GPS receiver 1412. For a given PNcode, for example, correlator 1418 may produce a correlation functiondefined over a range of code phases to set out a code phase searchwindow, and over a range of Doppler frequency hypotheses as illustratedabove. As such, an individual correlation may be performed in accordancewith defined coherent and non-coherent integration parameters.

Correlator 1418 may also be adapted to derived pilot-related correlationfunctions from information relating to pilot signals provided bytransceiver 1406. This information may be used by a subscriber stationto acquire wireless communications services.

Channel decoder 1420 may be adapted to decode channel symbols receivedfrom baseband processor 1408 into underlying source bits. In one examplewhere channel symbols comprise convolutionally encoded symbols, such achannel decoder may comprise a Viterbi decoder. In a second example,where channel symbols comprise serial or parallel concatenations ofconvolutional codes, channel decoder 1420 may comprise a turbo decoder.

Memory 1404 may be adapted to store machine-readable instructions whichare executable to perform one or more of processes, embodiments,implementations, or examples thereof which have been described orsuggested. CPU 1402 may be adapted to access and execute suchmachine-readable instructions. Through execution of thesemachine-readable instructions, CPU 1402 may direct correlator 1418 toperform dwells employing particular search modes at blocks 204 and 220,analyze the GPS correlation functions provided by correlator 1418,derive measurements from the peaks thereof, and determine whether anestimate of a location is sufficiently accurate. However, these aremerely examples of tasks that may be performed by a CPU in a particularembodiment and claimed subject matter in not limited in these respects.

In a particular embodiment, CPU 1402 at a subscriber station mayestimate a location the subscriber station based, at least in part, onsignals received from SVs during dwells as illustrated above. CPU 1402may also be adapted to estimate a location of such a subscriber stationand determine quantitative assessments of accuracy associated withpseudorange measurements such as RMSEs as illustrated above.Additionally, CPU 1402 may determine whether to terminate a process forimproving the accuracy of pseudorange measurements based, at least inpart, on such quantitative assessments of accuracy. Alternatively, asubscriber station may provide pseudorange measurements and quantitativeassessments of accuracy of same to a PDE (not shown). Such a PDE maythen estimate a location of the subscriber station based, at least inpart, on the pseudorange measurements. Further, as illustrated above inconnection with an alternative embodiment, such a PDE may also determinewhen to terminate a process of improving accuracy of a location estimateby performing a subsequent search dwell. It should be understood,however, that these are merely examples of systems for estimating alocation based, at least in part, on pseudorange measurements,determining quantitative assessments of such pseudorange measurementsand terminating a process to improve accuracy of pseudorangemeasurements according to particular embodiments, and that claimedsubject matter is not limited in these respects.

While there has been illustrated and described what are presentlyconsidered to be example embodiments, it will be understood by thoseskilled in the art that various other modifications may be made, andequivalents may be substituted, without departing from claimed subjectmatter. Additionally, many modifications may be made to adapt aparticular situation to the teachings of claimed subject matter withoutdeparting from the central concept described herein. Therefore, it isintended that claimed subject matter not be limited to the particularembodiments disclosed, but that such claimed subject matter may alsoinclude all embodiments falling within the scope of the appended claims,and equivalents thereof.

1-7. (canceled)
 8. A method comprising: determining a plurality ofpseudorange measurements to space vehicles (SVs) at a location based, atleast in part, on information received during a first dwell; determiningquantitative assessments of accuracy associated with said plurality ofpseudorange measurements; and selectively employing at least oneadditional dwell to improve accuracy of said pseudorange measurements inresponse to a weighting of at least some of said quantitativeassessments according to geometries associated with said SVs. 9-15.(canceled)
 16. An apparatus comprising: a computing platform to:determine a plurality of pseudorange measurements to space vehicles(SVs) at a location based, at least in part, on information receivedduring a first dwell; determine quantitative assessments of accuracyassociated with said plurality of pseudorange measurements; andselectively initiate at least one additional dwell to improve accuracyof said pseudorange measurements in response to a weighting of at leastsome of said quantitative assessments according to geometries associatedwith said SVs. 17-23. (canceled)
 24. An article comprising: a storagemedium having stored thereon instructions executable by a processor to:determine a plurality of pseudorange measurements to space vehicles(SVs) at a location based, at least in part, on information receivedduring a first dwell; determine quantitative assessments of accuracyassociated with said plurality of pseudorange measurements; andselectively initiate at least one additional dwell to improve accuracyof said pseudorange measurements in response to a weighting of at leastsome of said quantitative assessments according to geometries associatedwith said SVs. 25-31. (canceled)
 32. A position determination entitycomprising: a receiver to receive pseudorange measurements to spacevehicles (SVs) from a subscriber station over a data link, said positiondetermination entity being adapted to: determine a plurality ofpseudorange measurements to space vehicles (SVs) at a location based, atleast in part, on information received during a first dwell; determinequantitative assessments of accuracy associated with said plurality ofpseudorange measurements; and selectively initiate at least oneadditional dwell at said subscriber station to improve accuracy of saidpseudorange measurements in response to a weighting of at least some ofsaid quantitative assessments according to geometries associated withsaid SVs. 33-38. (canceled)
 39. A subscriber unit comprising: receiverto receive an acquisition assistance (AA) message from a data link, saidAA message comprising information indicative of geometries associatedwith space vehicles (SVs), said subscriber unit being adapted to:determine a plurality of pseudorange measurements to space vehicles(SVs) at a location based, at least in part, on information receivedduring a first dwell; determine quantitative assessments of accuracyassociated with said plurality of pseudorange measurements; andselectively initiate at least one additional dwell to improve accuracyof said pseudorange measurements in response to a weighting of at leastsome of said quantitative assessments according to said information. 40.The subscriber unit of claim 39, wherein the subscriber unit is furthercapable of determining the quantitative assessments of the accuracyassociated with the plurality of the pseudorange measurements based, atleast in part, on root mean square error estimates associated with thepseudorange measurements.
 41. The subscriber unit of claim 39, whereinthe subscriber unit is further capable of determining the weighting ofat least some of the quantitative measurements based, at least in part,on elevation angles associated with locations of the SVs.
 42. Thesubscriber unit of claim 39, wherein the subscriber unit is furthercapable of determining the weighting of at least some of thequantitative measurements based, at least in part, on azimuth anglesassociated with locations of the SVs.
 43. The method of claim 8, furthercomprising determining the quantitative assessments of the accuracyassociated with the plurality of the pseudorange measurements based, atleast in part, on root mean square error estimates associated with thepseudorange measurements.
 44. The method of claim 8, further comprisingdetermining the weighting of at least some of the quantitativemeasurements based, at least in part, on elevation angles associatedwith locations of the SVs.
 45. The method of claim 8, further comprisingdetermining the weighting of at least some of the quantitativemeasurements based, at least in part, on azimuth angles associated withlocations of the SVs.
 46. The method of claim 8, wherein the weightingof at least some of the quantitative assessments comprises a weightedhorizontal dilution of precision.
 47. The method of claim 8, furthercomprising processing signals received from at least some of the SVsover the first dwell and the at last one additional dwell.
 48. Themethod of claim 8, further comprising: combining the weightedquantitative assessments to provide a metric; and selectively initiatingthe at least one additional dwell in response to a comparison of themetric with a threshold value.
 49. The apparatus of claim 16, whereinthe computing apparatus is further capable of determining thequantitative assessments of the accuracy associated with the pluralityof the pseudorange measurements based, at least in part, on root meansquare error estimates associated with the pseudorange measurements. 50.The apparatus of claim 16, wherein the computing platform is furtheradapted to determine the weighting of at least some of the quantitativemeasurements based, at least in part, on elevation angles associatedwith locations of the SVs.
 51. The apparatus of claim 16, wherein thecomputing platform is further adapted to determine the weighting of atleast some of the quantitative measurements based, at least in part, onazimuth angles associated with locations of the SVs.
 52. The apparatusof claim 16, wherein the weighting of at least some of the quantitativeassessments comprises a weighted horizontal dilution of precision. 53.The apparatus of claim 16, wherein the computing platform is furtheradapted to process signals received from at least some of the SVs overthe first dwell and the at last one additional dwell.
 54. The apparatusof claim 16, wherein the computing platform is further adapted to:combine the weighted quantitative assessments to provide a metric; andselective initiating the at least one additional dwell in response to acomparison of the metric with a threshold value.
 55. The article ofclaim 24, wherein the instructions are further executable by theprocessor to determine the quantitative assessments of the accuracyassociated with the plurality of the pseudorange measurements based, atleast in part, on root mean square error estimates associated with thepseudorange measurements.
 56. The article of claim 24, wherein theinstructions are further executable by the processor to determine theweighting of at least some of the quantitative measurements based, atleast in part, on elevation angles associated with locations of the SVs.57. The article of claim 24, wherein the instructions are furtherexecutable by the processor to determine the weighting of at least someof the quantitative measurements based, at least in part, on azimuthangles associated with locations of the SVs.
 58. The article of claim24, wherein the weighting of at least some of the quantitativeassessments comprises a weighted horizontal dilution of precision. 59.The article of claim 24, wherein the instructions are further executableby the processor to process signals received from at least some of theSVs over the first dwell and the at last one additional dwell.
 60. Thearticle of claim 24, wherein the instructions are further executable bythe processor to: combine the weighted quantitative assessments toprovide a metric; and selectively initiate the at least one additionaldwell in response to a comparison of the metric with a threshold value.61. The position determination entity of claim 32, wherein the positiondetermination entity is further adapted to determine the quantitativeassessments of the accuracy associated with the plurality of thepseudorange measurements based, at least in part, on root mean squareerror estimates associated with the pseudorange measurements.
 62. Theposition determination entity of claim 32, wherein the positiondetermination entity is further adapted to process signals received fromat least some of the SVs over the first dwell and the at last oneadditional dwell.
 63. The position determination entity of claim 32,wherein the position determination entity is further adapted to: combinethe weighted quantitative assessments to provide a metric; andselectively initiate the at least one additional dwell in response to acomparison of the metric with a threshold value.
 64. An apparatuscomprising: means for determining a plurality of pseudorangemeasurements to space vehicles (SVs) at a location based, at least inpart, on information received during a first dwell; means fordetermining quantitative assessments of accuracy associated with theplurality of pseudorange measurements; and means for selectivelyinitiating at least one additional dwell to improve accuracy of thepseudorange measurements in response to a weighting of at least some ofthe quantitative assessments according to geometries associated with theSVs.
 65. The apparatus of claim 64, wherein the means for determiningquantitative assessments is capable of determining the quantitativeassessments of the accuracy associated with the plurality of thepseudorange measurements based, at least in part, on root mean squareerror estimates associated with the pseudorange measurements.
 66. Theapparatus of claim 64, further comprising means for determining theweighting of at least some of the quantitative measurements based, atleast in part, on elevation angles associated with locations of the SVs.67. The apparatus of claim 64, further comprising means for determiningthe weighting of at least some of the quantitative measurements based,at least in part, on azimuth angles associated with locations of theSVs.
 68. The apparatus of claim 64, wherein the weighting of at leastsome of the quantitative assessments comprises a weighted horizontaldilution of precision.
 69. The apparatus of claim 64, further comprisingmeans for processing signals received from at least some of the SVs overthe first dwell and the at last one additional dwell.
 70. The apparatusof claim 64, further comprising: means for combining the weightedquantitative assessments to provide a metric; and means for selectivelyinitiating the at least one additional dwell in response to a comparisonof the metric with a threshold value.